The invention is related generally to a system for position determination and more particularly, it is related to a system for determining the position of a syringe plunger and for providing an alarm upon the plunger reaching a selected position.
The infusion of fluids such as parenteral fluids into the human body is accomplished in many cases by means of a syringe pump having a lead screw on which a screw drive mechanism is mounted for translating the rotational motion of the lead screw into linear motion. A syringe plunger driver is connected to the screw drive mechanism for forcing the syringe plunger into the syringe barrel to expel the syringe contents at a desired rate. Because syringes having different sizes exist and syringes may be filled to different levels with infusion fluids, the extension of the plunger form the syringe barrel will differ from syringe to syringe. As a result, some screw drive mechanisms have a disengagement feature which the operator uses to disengage the screw drive mechanism from the lead screw, move the screw drive mechanism and plunger driver along the lead screw to the position of the new syringe plunger, engage the syringe plunger and then reengage the threads of the screw drive mechanism with the threads of the lead screw. Thus, the beginning position of the syringe plunger in relation to the lead screw will vary from syringe to syringe.
Likewise, the unextended or end-of-travel positions of the plungers in relation to the lead screw sill vary. Different syringes have different plunger extensions from the syringe barrel when the plunger has been fully driven into the syringe barrel and the syringe is empty. This distance is typically measured between the syringe barrel flange and the plunger flange and this extension distance varies between syringes of different sizes and also between syringes from different manufacturers. As a consequence, many pumps are designed to work only with one syringe from one manufacturer. This greatly reduces the value of the pump. If that particular syringe is not available, the pump is unusable.
In some syringe pumps, a split nut or half nut or other mechanism is used as part of the lead screw drive mechanism to selectively engage the lead screw. A linkage mechanism rigidly links the split nut to the syringe plunger driver, and because of this, the position of the syringe plunger can be determined from the position of the split nut or from other locations on the linkage.
In some syringe plunger position determination devices, the position of the plunger is determined by monitoring the number of revolutions that the lead screw rotated. A set of gears transmitting force from the motor to the lead screw is sued and the rotation of one of these gears is monitored. A disadvantage of this technique is the failure to monitor the actual device directly causing the syringe plunger movement, the plunger driver. In this technique, the monitored gear may turn regardless of the state of operation of the plunger drive. In the case where the threads of the lead screw are stripped or broken, no motion of the plunger driver may actually occur although the monitored gear turns. In such a situation, the patient may receive no infusion fluid until the pump operator notices that the contents of the syringe have not decreased.
In other pumps, markers at fixed positions along the travel of the plunger driver are used. These may be optical markers and are read by a detector on the plunger driver. Other systems have been used, including magnetic and inductive systems. However, in most of these systems, only relative position information concerning the plunger driver is available. The system can determine how far the plunger driver has moved but cannot determine where it is in relation to a fixed position, such as its end of travel or unextended position.
In another syringe plunger position device, an electric contact is made between the movable syringe plunger driver and a stationary linear potentiometer by way of an electrically conductive wiper mounted on the plunger driver. The position of the syringe plunger is then ascertained by comparing the voltage of the wiper to other voltages across the potentiometer. The wiper voltage is proportional to the position of the syringe plunger. This approach, however, has the disadvantage of having an exposed electrical contact at the wiper which can accumulate contaminants and degrade performance. Sealing the wiping contact is difficult due to its connection to the plunger driver and the relatively long distance it moves.
In addition, it is believed by the inventor that the above approaches have not provided a means to accept various sizes of syringes with their differing plunger travel distances and the differing positions of their plungers when unextended from the syringe barrel.
Hence those skilled in the art have recognized a need for a more accurate, versatile and reliable plunger position determination system. The present invention fulfills that need.